The present invention relates to a hydraulic system and more particularly relates to hydraulic systems including one or more variable displacement pumps having their displacements controlled automatically in response to the requirement of various hydraulic functions as indicated by power beyond flow emanating from control valves for the various functions.
Power beyond is a typical option available on most valves used in open center or constant flow hydraulic systems. With a plurality of control valves connected in series, this option gives the first control valve priority on the hydraulic flow available and when the flow is not used it is directed out the power beyond port to the next valve rather than back to the hydraulic reservoir as is done with conventional open center valves.
The most common open center power beyond valves use open center spools for function control. The spools are moved to restrict the flow through the open center passage causing a pressure increase to the load pressure. The flow is divided between the open center passage and the work ports with the open center flow being directed out the power beyond port and the returning load flow being directed back to sump. Dividing flow in this manner makes it difficult for an operator to control the speed of a function since fluctuations in function load must be compensated for by spool movement.
This problem of control is somewhat alleviated by a more specialized type of open center, power beyond valve which incorporates a pressure compensated flow control valve which operates to divide flow in response to the demand for fluid of a function controlled by the valve. Flow is related to spool movement with the flow being maintained constant for varying function loads and also being limited to a predetermined rate. Examples of pressure compensated, open center, power beyond valves are found in U.S. Pat. No. 3,455,210 issued to Allen on July 15, 1969; U.S. Pat. No. 3,465,519 issued to McAlvay et al on Sept. 9, 1969; and U.S. Pat. No. 3,718,159 issued to Tennis on Feb. 27, 1973.
For the sake of efficiency, systems employing open center valves use variable displacement pumps which are automatically controlled in some way to meet the instantaneous demand of the systems. One example of a system employing a variable displacement pump controlled in this manner is disclosed in the aforementioned McAlvay et al patent. Specifically, McAlvay et al disclose a system employing a single variable displacement pump, a multiplicity of functions and control valves therefore with the power beyond flow from the last control valve being coupled to a pressure responsive displacement controller for decreasing the output of the pump in response to increasing power beyond flow.
The McAlvay et al system suffers from the disadvantage that it does not make provision for having functions of equal priority connected in parallel to a common source of fluid pressure or for situations where a second pump is needed for supplying the maximum possible demand that the functions might have for fluid.